The present invention generally relates to nondestructive inspection methods and, more particularly, to detecting substructure using precision eddy current scanning.
Automated assembly systems in the aerospace industry, for example, for airframe assembly of aircraft, generally employ some type of vision system for locating structure components and key features of components, such as edges of flanges, machined steps, and tooling holes. Knowledge of the exact location of these features is necessary, since these features are used to adjust numerically controlled programs for drilling holes or other machining operations, such as trimming or reaming, to maintain blueprint tolerances. Currently, it is often necessary to manually record where the substructure is located. In order to do this, the outer mold line skins, for example of a section of the fuselage or the wing, need to be removed to make the substructure underneath visible. Once a map of the substructure is created, the outer mold line skins are temporarily fastened to the structure and the created map of the substructure needs to be transferred to the skin. Since this step is performed while the assembly is in the machine bed, the flow time is impacted and the percentage of the machine time actually used for the intended function, such as drilling, is reduced.
Eddy current as a nondestructive inspection process is commonly used in the aerospace industry to detect subsurface flaws or anomalies in conductive materials. The advantage of eddy current for nondestructive inspection is the ability to perform scanning through the outer skin material. Eddy current data can be collected using automated scanning systems to improve the quality of the measurements and to construct images of scanned areas. The most common type of scanning is line scanning where an automated system is used to push the probe at a fixed speed. The data is usually presented as a strip chart recording. The advantage of using a linear scanning system is that the probe is moved at a constant speed such that an indication on the strip chart can be correlated to a position on the part being scanned. Two-dimensional scanning systems are used to scan a two-dimensional area. This could be a scanning system that scans over a relatively flat area in an x-y raster mode. The data is typically displayed in a C-scan, which is a false-color plot of signal strength or phase angle shift as a function of position. Mobile automated scanners, such as MAUS® IV and V developed by The Boeing Company, St. Louis, are generally used in the aerospace industry for nondestructive testing utilizing eddy current and ultrasonic waves. MAUS IV eddy current C-scans are used, for example, for corrosion detection or crack detection around fastener holes.
As can be seen, there is a need for a method to accurately and effectively locate and map the substructure features of an aircraft airframe that are located underneath the outer mold line skins. Furthermore, there is a need to eliminate the step of the outer skin removal in order to see the substructure and the step of skin installation after recording the substructure. Still further, there is a need to improve the product flow and automation of aircraft assemblies.
There has, therefore, arisen a need to provide a method for detecting substructure using nondestructive techniques. There has further arisen a need to locate substructure features with sufficient accuracy to control assembly operations and meet engineering tolerances. There has still further arisen a need to provide a device that allows detection and location of substructure features within the tolerances required.